98 THE PRINCIPLES OF AGRICULTURE 



results, particularly when it is desired to improve 

 the texture of clay soils (58, 58a) . 



153. A soil may be tested to determine if it is 

 acid by placing a piece of blue litmus paper 

 (kept at drug stores) against the moist soil. If 

 the paper reddens and remains so after drying, it 

 shows the presence of an acid in the soil. It is 

 best to apply the paper not to the top of the soil, 

 but to the side of a hole such as would be made 

 by inserting a spade and moving it to and fro. 



6. Commercial Fertilizers 

 6a. What they are 



154. Under the name of commercial fertilizers, 

 one can buy the various forms of nitrogen, phos- 

 phoric acid and potash. These elements may be 

 purchased singly or mixed in any combination. 

 A fertilizer containing all three is called a com- 

 plete manure or fertilizer. In buying, one should 

 be guided by the guaranteed analysis and not by 

 any particular name or brand. 



155. The commercial value of nitrogen is 

 about three times that of either phosphoric acid 

 or potash, which are approximately 5 cents per 

 pound. The prices of these elements may vary, 

 but the following will serve as an illustration of 

 the computing of relative values of different fer- 



